1. Field of the Invention (Technical Field)
The present invention relates to spectroscopy, in particular to laser modulation spectroscopy.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Absorption spectrometry is a widely used method for measuring the presence or concentration of chemical compounds. Modulation techniques have been widely used to improve the sensitivity of absorption spectrometers. In these techniques, the optical frequency or wavelength of a light source is rapidly varied, or modulated. The modulated light passes through a sample (usually a gas at or below atmospheric pressure) onto a photodetector. The output of the photodetector is amplified as needed, then directed to one or more stages of demodulation or synchronous detection. The demodulation step multiplies the signal from the photodetector by a sine wave whose frequency is related to the modulation frequency or frequencies. The demodulation step is usually implemented with a lock-in amplifier or a radio frequency mixer followed by a low pass filter. Modulation techniques improve sensitivity, in part by exploiting the wavelength dependence of the spectrum of the compound under study and in part because the lasers that are used as light sources typically have less noise at high frequencies. Such noise typically follows a 1/f distribution, where f is the measurement frequency. While modulation techniques can be implemented using an external modulator such as an electro-optic modulator, they are most easily implemented when the light source is a diode laser. Modulating the current used to operate the diode laser modulates its wavelength.
Modulation techniques include wavelength modulation spectroscopy (WMS), frequency modulation spectroscopy (FMS), tone burst spectroscopy (TBS), and two-tone frequency modulation (TT-FMS). These modulation techniques have been described in many publications, for example Pavone et al., “Frequency- and wavelength-modulation spectroscopies: comparison of experimental methods using an AlGaAs diode laser,” Applied Physics B, 56, 118–122 (1993); J. A. Silver, “Frequency Modulation Spectroscopy for Trace Species Detection: Theory and Comparison Among Experimental Methods,” Applied Optics 31, 707–717 (1991); P. Kluczynski et al., “Theoretical description based on Fourier analysis of wavelength-modulation spectrometry in terms of analytical and background signals,” Applied Optics 38, 5803–5815 (September 1999); G. Bjorklund, Method and Device for Detecting a Specific Spectral Feature, U.S. Pat. No. 4,297,035 (1981); T. Gallagher et al., Frequency Modulation Spectroscopy Using Dual Frequency Modulation and Detection, U.S. Pat. No. 4,765,736 (1988); D. S. Bomse, Applied Optics 30, 2922–2924; J. Reid and D. Labrie, “Second Harmonic Detection with Tunable Diode Lasers Comparison of Experiment and Theory,” Applied Physics B 26, 203–210 (1981).
In the standard wavelength modulation spectroscopy (WMS) approach as applied to diode laser spectroscopy, the laser drive current is modulated with a sine wave. A lock-in amplifier referenced to exactly twice the modulation frequency is used to process the signal from the photodetector. By slowly scanning the laser wavelength, a portion of the absorption spectrum can be recorded. The output of the lock-in amplifier is approximately the second derivative of the absorption spectrum. The peak signals are strongest when the wavelength excursion of the laser is made to be about equal to the width of the spectral features of the gas to be detected, and the exact relationship between signal strength and modulation depth has been worked out for a number of typical spectral line shapes. When, instead, the modulation frequency is made to be about equal to the spectral width, the technique is called frequency modulation spectroscopy (FMS).
In the tone burst spectroscopy method, the light from the laser (or other narrow band source) is transmitted through a sample and onto a detector that produces an electrical output proportional to the transmitted power. The laser's optical frequency or wavelength is modulated at a tone frequency F1 and the modulation is turned on and off at some lower, burst frequency F2. The output of the photodetector, suitably amplified, is measured with a lock-in amplifier referenced to the frequency F2. The output of the lock-in is a measure of the difference in transmission with and without the modulation. Good detection sensitivity is achieved when the modulation depth or modulation frequency is about equal to the width of the targeted spectral transition. Detection at F2 avoids laser 1/f noise at still lower frequencies. Tone burst spectroscopy has been developed by a number of workers. H. M. Pickett, “Determination of collisional linewidths and shifts by a convolution method,” Applied Optics 19, 2745–2749 (1980), first applied the tone burst method to absorption spectroscopy in microwave experiments. C. S. Gudeman et al., “Tone-burst Modulated Color-center-laser Spectroscopy,” Optics Letters, Vol. 8, pp. 310–312 (1983), demonstrated the use of tone burst spectroscopy with a color center laser. H. Sassada et al., “Ti-Sapphire Laser Spectrometer for Doppler-limited Molecular-Spectroscopy,” J. Opt. Soc. Amer. B Vol. 11, pp. 191–197 (1994), notes the close connection between tone burst spectroscopy and two-tone frequency modulation spectroscopy. TTFMS is a variation of FMS that was developed to avoid the need for extremely high frequency detectors and demodulators required for FMS. Two high modulation frequencies are used, F3 and F4, while the demodulation step is referenced to the lower difference F3–F4 between the frequencies. Plotting the TTFMS modulation signal vs. time reveals the close relation of TTFMS to TBS. The high frequency modulation amplitude rises and falls in a slower envelope. The frequency of the envelope is F3–F4. Thus, TTFMS can be thought of as a form of TBS, but with a smooth envelope instead of the on-off step function typically used in TBS.
In practical systems based on WMS, FMS, TBS or TTFMS, the sensitivity is limited by optical noise arising from light scattered or reflected within the optical system. This light coherently beats against the main optical beam at the detector. When a section of the spectrum is scanned, this noise appears as a sinusoidal modulation of the baseline intensity. It is known as an interference fringe or etalon. The period of the fringe depends on the extra time taken for the stray reflection to reach the detector. When measured in units of the spectral wavelength or frequency of the laser it is known as the free spectral range. The amplitude of the fringe depends on the strength of the reflected beam and its overlap with the main beam. By careful design of the optical system, it is possible to reduce the amplitude of such fringes to about 0.001% of the laser intensity. The phase of the fringe is sensitive to small changes in the optical alignment, so it usually varies when the ambient temperature changes.
Although the effect of the fringes is small, so low is the noise from a diode laser spectrometer that one or two orders of magnitude improvement in sensitivity could be achieved if it were possible to suppress them. As a result, several mechanical approaches have been taken to suppress such noise. One can vary the time delay of the stray beam by vibrating the position of an optical element [J. A. Silver and A. C. Stanton “Laser Absorption Detection Enhancing Apparatus and Method,” U.S. Pat. No. 4,934,816 (1990)] or by varying the angle of a window [C. R. Webster, “Brewster-Plate Spoiler: a Novel Method for Reducing the Amplitude of Interference Fringes that Limit Tunable Diode Laser Absorption Sensitivities,” J. Optical Society of America B 2, 1464–1470 (1985)], then average over the various delays. When the time delay is of the order of a half optical cycle or more of the light, the phase of the fringe varies by 180 degrees or more and these methods are effective. However, these approaches require mechanically changing the position of an optic, incurring the cost of an actuator. One can vary the pressure of the sample [A. Fried et al., “Reduction of Interference Fringes in Small Multipass Absorption Cells by Pressure Modulation,” Applied Optics 29, 900–902 (1990)], which may act to vary the delay either mechanically or by changing the index of refraction. Alternatively, the sample dependent part of the signal can be extracted by suitable analysis of its pressure dependence. This approach involves an actuator to vary the sample pressure and requires a pumping system to add or remove gas. As it is difficult to rapidly vary the pressure, this approach may not be suitable when a fast response is desired from the instrument.
Variations of both the WMS and FMS approaches have been developed to reduce optical noise. D. S. Bomse et al., “Dual-Modulation Laser Line-Locking Technique For Wavelength Modulation Spectroscopy,” U.S. Pat. No. 6,351,309 (2002); D. S. Bomse, “Dual-Modulation Laser Line-Locking Scheme”, Applied Optics, 30 (1991) reported an approach in which two modulation frequencies are added to the bias current, and the signal is demodulated sequentially at a harmonic of each. E. A. Whittaker et al., “Method and apparatus for reducing fringe interference in laser spectroscopy”, U.S. Pat. No. 5,267,019 (1993) and “Method and apparatus for dual modulation laser spectroscopy,” U.S. Pat. No. 5,636,035 (1997) reported another approach. These approaches have the drawback that the phases of two demodulation steps must be adjusted to achieve high sensitivity and calibration stability. They have the further drawback that the peak modulation amplitude can be twice as large as for standard WMS or FMS.
D. T. Cassidy and J. Reid, “Harmonic Detection with Tunable Diode Lasers—Two-Tone Modulation,” Applied Physics B 29, 279–285 (1982) employ WMS with the addition of a second modulation frequency, but without a second demodulation step. The amplitude of this second modulation, termed a jitter modulation, is chosen to minimize the detection of particular fringe, and it may be quite small. They show that the detection sensitivity for a given fringe depends on the amplitudes of each modulation waveform and on the period of the fringe, in particular as the product of two Bessel functions. The amplitude of the first modulation is adjusted to maximize detection of the target absorption signal, while the amplitude of the second modulation is chosen to minimize detection of the interfering optical fringe. This approach is especially useful when the free spectral range of the fringe is smaller than the width of the spectral feature to be detected. A disadvantage of this approach is that the free spectral range of the fringe must be known, and it is not useful for eliminating more than one fringe.
The use of non-sinusoidal modulation waveforms in WMS reduces the sensitivity to fringes. T. Iguchi, “Modulation Waveforms for Second Harmonic Detection with Tunable Diode Lasers,” J. Optical Society of America B, 3, 419–423 (1986), compared sine wave modulation, square wave modulation, quasi-square wave modulation, triangular wave modulation, and an inverse integral raised cosine (IIRC) waveform. The best waveform is one that is sensitive to the Fourier components of the signal but insensitive to fringes outside this range. Generally, the waveforms with sharp tips showed less sensitivity to etalons with small free spectral range. Triangle waves give a good response and the IIRC waveform is even better. However, Iguchi noted that the IIRC waveform extends to infinity, so it is impractical for a realistic instrument. Iguchi also considered the use of jitter waveforms of various shapes together with the various modulation waveforms. Adding a jitter modulation produces a weighted average or blurring of the spectrum, with the weighting function determined by the jitter amplitude and shape. The jitter approach was found not to work when the free spectral range of the fringe was comparable to or larger than the line width of the signal.
The present invention provides a means of minimizing fringes over a wide range of free spectral ranges, while optimizing the detection of a target signal. The present invention also provides a method to compute modulation waveforms that minimize the detection of fringes or that optimize the detection of signals of arbitrary shape.